While ammonia oxidation has been widely studied, nitrite oxidation is still not well understood. To study the effect of organics on nitrite oxidation, one control group and four treatment groups were designed with sodium acetate (C) to nitrite (N) ratios of 0, 0.44, 0.88, 4.41, and 8.82. Normal nitrite oxidization reactions were performed when C/N=0, but nitrite oxidization and partial denitrification occurred with low C/N ratios (0.44 or 0.88). At high C/N ratios (4.41 or 8.82), we mainly observed denitrification. In contrast to C/N=0, the nitrite oxidization rate was unaffected when C/N=0.44, but decreased with C/N=0.88. The structure of bacterial communities varied significantly between autotrophic and mixotrophic conditions. Nitrobacter was hard to detect by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) while heterotrophs and especially denitrifiers were in the majority under mixotrophic conditions. Real-time PCR indicated that the Nitrobacter population decreased from 2.42 x 10(4) to 1.34 x 10(3) 16S rRNA gene copies/ng DNA, while the quantity of denitrifiers obviously increased from 0 to 2.51 x 10(4) nosZ gene copies/ng DNA with an increasing C/N ratio. The degree of denitrification differed between C/N=4.41 and C/N=8.82, as indicated by nitrite-N and nitrate-N curve analysis, as well as by the apparent bacterial community structure. Our findings provide critical insight regarding the relationship between organics and nitrification in biofilms.